EPDM rubber, an elastic terpolymer of ethylene, an alpha-olefin such as propylene, and a diene such as ethylidene norbornene, has the molecular structure not including an unsaturated bond in the main chain and displays superiority to general conjugated diene rubbers in regards to weather resistance, chemical resistance, heat resistance, and so forth. Due to the characteristics, the elastic terpolymer such as EPDM rubber has been widely used for industrial materials, such as materials for all sorts of automobile parts, electric wires, hoses for construction or other use purposes, gaskets, belts, bumpers, a blend with plastics, etc.
The elastic terpolymer like EPDM rubber has been prepared mostly by copolymerizing three different monomers in the presence of a catalyst including a vanadium compound, such as a vanadium-based Ziegler-Natta catalyst. However, since the vanadium-based catalyst has low catalytic activity, there is a need to use an excess of the catalyst, thereby adversely increasing the content of the residual metals in the copolymer. It is therefore necessary to add the process for catalyst elimination and decolorization and also a low-temperature polymerization process, in which case the reaction temperature is hard to control. The truth is that it is disadvantageously not easy to control the uptake rate of the comonomers such as propylene and diene and to prepare an elastic copolymer with high molecular weight and high viscosity. Hence, in the case of using the vanadium-based catalyst, there has been a limitation on the preparation of the elastic terpolymer with various properties. To overcome this problem, there has recently been developed a method of preparing an elastic terpolymer like EPDM rubber by using a metallocene-based group 4 transition metal catalyst in place of the vanadium-based Ziegler-Natta catalyst.
Such a group 4 transition metal catalyst shows high polymerization activity in the polymerization of olefins, and makes it possible not only to prepare the copolymer having higher molecular weight but also to control the molecular weight distribution and the composition of the copolymer with ease. Furthermore, it has an advantage of enabling the copolymerization of different comonomers. For example, U.S. Pat. Nos. 5,229,478 and 6,545,088, and Korea Pat. No. 0,488,833 disclose that an elastic terpolymer having a high molecular weight can be obtained with excellent polymerization activity by using various metallocene-based group 4 transition metal catalysts prepared from ligands, such as cyclopentadienyl, indenyl, fluorenyl, and so on.
However, the use of the conventional group 4 transition metal catalyst for copolymerization of three different monomers has a disadvantage in that the repeating units derived from the individual monomers are unevenly distributed in the chain of the copolymer because of high reactivity to the comonomer of alpha-olefin. For this reason, it is in fact difficult to obtain an elastic terpolymer such as EPDM rubber with excellent elasticity and flexibility.
Accordingly, there has been a sustained demand for developing an elastic terpolymer with good mechanical properties and elasticity (flexibility) at the same time and a method for preparing the same with high productivity and yield.